In optical communications, wavelength division multiplexing (WDM) is used for increasing the transmission capacity of a single optical fibre. The vast expansion in demand for communications bandwidth is pushing fibre transmission technology to its physical limits. Increased transmission capacity is obtained by increasing the transmission data rate through the fibre. Current transmission data rates are approaching the limits of opto-electronic components. Increased transmission capacity is also obtained by reducing the channel frequency spacing in a WDM optical fibre link. Such WDM links are referred to as DWDM (Dense Wavelength Division Multiplexing) because the density of the wavelength channels per unit wavelength is higher than in conventional WDM. The use of both higher data rates and closer channel spacing puts severe demands on the performance of the optical demultiplexer which can limit the optical capacity of the transmission link. For example, in current WDM links, the optical signals are transmitted at 10 Gbit/s on wavelength channels spaced at frequency intervals of 100 GHz. It is planned to increase the data rate to 40 Gbit/s, which concomitantly increases the bandwidth of the wavelength channel. The result is that in the wavelength domain, the linewidths of adjacent wavelength channels overlap making it difficult to demultiplex the individual wavelength channels without incurring an unacceptable loss in data information.
To better appreciate the problems consider the following. As the modulation bit rate is increased from 10 GHz to 40 GHz, the bandwidth ΔB of a channel signal increases; the relationship between the light pulse width Δt and signal bandwidth ΔB is Δt=1/ΔB. In the case of a 40 GHz signal using a RZ (return to zero) modulation format, the separation between the pulses is 25 ps, but the pulse width, At, is only 12.5 ps and consequently, the modulation bandwidth ΔB is 80 GHz. The minimum modulated bandwidth of these signals is 60 GHz. Thus theoretically, wavelength channels transmitting data at 40 GHz could be multiplexed together onto a DWDM link in which the frequency separation is only 100 GHz. However, the theoretical capacity of the WDM link is limited by the band pass and the environmental stability of the optical filters used in demultiplexing. The available filter pass-band is determined by the figure of merit, which is defined as the bandwidth at the 0.5 dB point divided by the bandwidth at the 25 dB point and this is typically 0.4 to 0.5. Thus a high quality optical filter that could be used to demultiplex the DWDM signals have a measured pass bandwidth of 50 GHz and net bandwidth of only 25 GHz. Misalignment of the filter wavelength to the signal wavelengths due to manufacturing tolerances and environmental factors such as ageing will reduce the available bandwidth even further. Thus, demultiplexing this optical signal using prior art techniques would result in unacceptable error rates and data loss.
It is an object of the invention to provide a demultiplexer capable of demultiplexing high bit-rate optical signals.
It is a further object of the invention to provide a demultiplexer capable of demultiplexing optical signals in which the linewidths of adjacent channels overlap in the wavelength domain.
It is a further object of the invention to provide a low loss high bit-rate demultiplexer.
It is another object of the invention to provide a high bit-rate demultiplexer having a low error rate.